The subject invention relates to anionic polymerization resulting in diene polymer and copolymer elastomers. More particularly, the invention relates to lithium amino magnesiate initiators which are stable at high polymerization temperatures and produce polymers containing a high level of tertiary amine functionality, and elastomers exhibiting reduced hysteresis properties.
When conducting polymerization on a commercial basis, it is desirable to increase the efficiency of polymerization reactions by increasing the number of monomers which can be incorporated into the polymer chain in a given time period. This may be accomplished by utilizing high temperatures during polymerization. When employing anionic polymerization initiators, such a system requires that the initiators be stable at high temperatures and capable of producing chain-end functionalized polymers, having a narrow molecular weight distribution, which can be compounded to produce elastomers exhibiting reduced hysteresis properties. Such elastomers, when compounded to form articles, such as tires, power belts and the like, will show an increase in rebound, a decrease in rolling resistance and have less heat build-up when mechanical stresses are applied.
Previous attempts at preparing reduced hysteresis compounds have included high temperature mixing of the filler-rubber mixtures in the presence of selectively-reactive promoters to promote compounding material reinforcement, surface oxidation of the compounding materials, and chemical modifications to the terminal end of polymers using tetramethyldiamino-benzophenone (Michler's ketone), tin coupling agents and the like, and surface grafting thereon. All of these approaches have focused upon increased interaction between the elastomer and the compounding materials.
It has also been recognized that carbon black, employed as a reinforcing filler in rubber compounds, should be well dispersed throughout the rubber in order to improve various physical properties. This dispersion can be achieved, for instance, by end capping polydienes by reacting a metal terminated polydiene with a capping agent, such as a halogenated nitrile, a heterocyclic aromatic nitrogen-containing compound or an alkyl benzoate. Additionally, it is known in the art that both ends of the polydiene chains can be capped with polar groups by utilizing functionalized anionic initiators, such as lithium amides.
The present invention provides novel initiators for anionic polymerization, to form elastomers with functional groups derived from said initiators. The functional groups are incorporated at the polymer chain ends, providing improved dispersability of carbon black throughout the elastomeric composition during compounding. As will be described herein below, the functional groups are cyclic amine substituents of the initiators which comprise lithium amino magnesiate complexes. In the polymer, the cyclic amine substituent is incorporated at one end of the polymer chain and the magnesium atom is complexed to the lithium atom and the magnesium-lithium complex is carried at the other ("living") end of the polymer chain prior to quenching.
Lithium amino initiators containing functionalizing agents, such as substituted aldimines, ketimines and secondary amines, are known in the art to produce low hysteresis rubbers. It is also known in the art that magnesium dihydrocarbyl compounds, while not by themselves effective polymerization initiators for diene and styrene polymerization, can participate in polymerization when complexed either with an alkyl lithium initiator or with the propagating polymer-lithium molecules. Magnesium dihydrocarbyl compounds have been utilized, in combination with alkali metal compounds, such as lithium, sodium and potassium alkyl sulfides, amines, amides and acetylides, to produce anionic type initiators.
However, when lithium amide initiators are used at high polymerization temperatures it is difficult to maintain the "living" ends or the polymer-lithium bonds needed for efficient polymerization and termination reactions. With known initiators it has been found that the lithium constituent will often be involved in metalation reactions or combine with an available alpha-hydrogen atom, resulting in lithium hydride, especially at elevated temperatures, thereby destroying the initiator and causing additional harmful side reactions. Hence, high temperature polymerizations have proven to be difficult to maintain and difficult to terminate efficiently.
A need exists, therefore, for an improved polymerization initiator which, when employed in an anionic polymerization, will result in polymers with chain ends having functional groups derived from the initiator. Further, a need exists for such an initiator that will perform effectively at high polymerization temperatures resulting in narrow molecular weight distribution polymers and the retention of "living" ends.